Diseases involving the skin and underlying soft tissues are legions. These tissues are involved by a wide range of congenital, inflammatory, infectious, traumatic, and neoplastic processes. Aside from physical examination, the diagnosis of most of these lesions is invasive, time- consuming, and costly, often requiring surgical excision or biopsy followed by pathological investigations. A non-invasive, efficient, low cost technique of evaluating skin and soft tissue cellular changes would be extremely valuable in the management of numerous conditions. It is known that normal nuclei have a characteristic diameter between 4 and 7mum, while dysplastic nuclei generally enlarge and can be as large as 20 mum. Thus, enlarged nuclei are primary cellular indicators of cancer, dysplasia and cell regeneration in most human tissues. Current methods such as image analysis and flow cytometry used to size nuclei in tissue require dispersed cellular samples in liquid media or tissue sections which cannot be used for in vivo analysis of cells. This R03 application is aimed to develop a novel optical method that will allow us to extract in vivo nuclear size distribution directly from multiply scattered light or photon migration measurement in tissue. Our idea for nuclear sizing is to use model-based computational algorithms to reconstruct the nuclear size distribution that is contained in the tissue scattering spectral. Our measurement of nuclear size distribution can be provided in real-time. Our approach will also allow us to recover the size distributions of other scatterers such as blood cells. With this capability, we will be able to detect abnormalities due to the size changes of these other substructures. Thus, the specific aims of this application are: (1) to continue the development of regularized inverse algorithms for the extraction of nuclear size distribution, (2) to evaluate and optimize the integrated functioning of the computer algorithms and a CD-based continuous-wave optical spectral system using tissue phantom, cultured cell and the human skin equivalent experiments, and (3) to evaluate our integrated optical system based on preliminary data obtained during a human subjects trial.